This project will provide information about timing of RNA and protein syntheses necessary for the expression of specific neuronal phenotypes during development. We have described the sequence of appearance of some cellular properties of amphibian spinal neurons differentiating in vivo. The same developmental changes occur in embryonic spinal neurons dissociated and grown in culture. Specific metabolic inhibitors will be added to the culture media at later and later stages of development, and the time of application at which their continuous presence no longer blocks expression of electrical excitability, chemosensitivity and high affinity neurotransmitter uptake will be determined. Brief applications of reversible metabolic inhibitors will provide information about the existence of critical periods, during which expression of single phenotypes may be blocked. If such critical periods exist, it will be possible to examine the consequences of the absence of one phenotype on the expression of others.